CVE-2026-30226: Svelte Devalue Prototype Pollution DoS

CVE-2026-30226 Overview

Svelte devalue is a JavaScript library that serializes values into strings when JSON.stringify isn't sufficient for the job. A prototype pollution vulnerability has been discovered in devalue versions 5.6.3 and earlier. The devalue.parse and devalue.unflatten functions are susceptible to prototype pollution via maliciously crafted payloads. Successful exploitation could lead to Denial of Service (DoS) or type confusion within affected applications.

Critical Impact

Applications using affected versions of the devalue library may be vulnerable to DoS attacks or type confusion when processing untrusted serialized data through the devalue.parse or devalue.unflatten functions.

Affected Products

• Svelte devalue versions ≤ 5.6.3

Discovery Timeline

• March 11, 2026 - CVE-2026-30226 published to NVD
• March 12, 2026 - Last updated in NVD database

Technical Details for CVE-2026-30226

Vulnerability Analysis

This vulnerability is classified as Prototype Pollution (CWE-1321), a type of injection flaw specific to JavaScript environments. Prototype pollution occurs when an attacker is able to modify the prototype of a base object, which then affects all objects that inherit from that prototype. In the case of CVE-2026-30226, the vulnerability exists in the deserialization functions devalue.parse and devalue.unflatten.

The devalue library is designed to handle serialization of complex JavaScript values that cannot be properly serialized with standard JSON.stringify. This includes handling of objects with circular references, typed arrays, Maps, Sets, and other complex data structures. The parsing functions must reconstruct these objects from serialized strings, and this reconstruction process is where the vulnerability manifests.

Root Cause

The root cause of this vulnerability lies in insufficient validation of object property assignments during the deserialization process. When devalue.parse or devalue.unflatten processes a maliciously crafted payload, it fails to properly sanitize property keys before assignment. This allows an attacker to inject properties like __proto__, constructor, or prototype that can modify the base Object prototype, affecting all subsequent objects in the JavaScript runtime.

Attack Vector

The attack vector is network-based, requiring an attacker to supply a malicious serialized payload to an application that processes it using the vulnerable devalue.parse or devalue.unflatten functions. The attack complexity is considered high as exploitation requires the attacker to craft a specific payload and find an application endpoint that processes untrusted serialized data.

A successful prototype pollution attack could result in:

• Denial of Service: By polluting the prototype with properties that cause runtime errors or infinite loops
• Type Confusion: By modifying expected object properties, causing applications to misinterpret data types and potentially bypass security checks

The vulnerability is exploited by sending a specially crafted serialized object that, when parsed by the devalue library, injects malicious properties into the Object.prototype. Technical details and proof-of-concept information can be found in the GitHub Security Advisory.

Detection Methods for CVE-2026-30226

Indicators of Compromise

• Unexpected properties appearing on JavaScript objects that were not explicitly defined
• Application crashes or errors related to prototype chain manipulation
• Unusual behavior in functions that iterate over object properties using for...in loops
• Runtime errors indicating type confusion or unexpected property types

Detection Strategies

• Audit application dependencies to identify usage of devalue library versions ≤ 5.6.3
• Implement runtime monitoring for modifications to Object.prototype or other built-in prototypes
• Review application logs for errors related to unexpected object property access
• Deploy Software Composition Analysis (SCA) tools to detect vulnerable library versions in your codebase

Monitoring Recommendations

• Monitor npm/yarn lock files for the presence of vulnerable devalue versions
• Implement Content Security Policy (CSP) headers where applicable to limit script execution contexts
• Set up alerts for dependency vulnerability notifications from package registries
• Conduct regular security audits of JavaScript serialization/deserialization code paths

How to Mitigate CVE-2026-30226

Immediate Actions Required

• Upgrade the devalue library to version 5.6.4 or later immediately
• Review application code for any usage of devalue.parse or devalue.unflatten with untrusted input
• Implement input validation on any data processed by devalue functions
• Consider using Object.freeze() on critical objects as a defense-in-depth measure

Patch Information

The Svelte team has released version 5.6.4 of the devalue library which addresses this prototype pollution vulnerability. Users should update their package dependencies immediately.

For npm users:

npm update devalue

For yarn users:

yarn upgrade devalue

Verify the update by confirming your package-lock.json or yarn.lock contains devalue version 5.6.4 or higher. Additional details are available in the GitHub Security Advisory.

Workarounds

• Avoid processing untrusted input through devalue.parse or devalue.unflatten until patched
• Implement server-side validation to sanitize incoming data before deserialization
• Use Object.create(null) for objects that will hold untrusted data to avoid prototype chain exposure
• Consider implementing a security wrapper around devalue functions that sanitizes dangerous property keys

# Verify your devalue version
npm list devalue

# Update to the patched version
npm install devalue@5.6.4

# Audit for other vulnerable dependencies
npm audit